So far, lithium ion batteries have been widely used in the fields of various mobile electronic products and electric tools. The cathode material is a key to improving the energy density and the safety and reducing the cost in lithium ion batteries. Further improving the power density, the energy density and the safety performance of the material is the recent direction of the development in cathode materials for lithium ion batteries.
Among many candidate materials alternative to LiCoO2, LiNiO2 has drawn great attention due to its high capacity, low cost and low pollution. LiNi1-xCoxO2, in addition to its advantages of high specific capacity and low cost in comparison with LiNiO2, has a relatively good cycling performance. LiNi1-xCoxO2 can show a specific capacity of 190 mAh/g. However, the stability of the nickel-cobalt binary material still cannot meet the current requirements of 3 C (consumer electronics, computer, communication) electronic products and power batteries on the materials.
Bulk doping and surface coating are the most major methods for improving the stability of nickel-based materials. As a nickel-cobalt-aluminum material representing aluminum-doped materials, LiNi1-x-yCoxAlyO2 is an isomorphic solid solution of LiNiO2, LiCoO2 and LiAlO2. It has advantages including high energy density, good thermal stability, low cost, and environmental friendliness, and has become a high-end energy-storage material in the fields of 3 C and power batteries. However, due to the thermodynamic instability of trivalent nickel, the NCA (lithium nickel cobalt aluminate material) is difficult to be synthesized. Divalent nickel ions are difficult to be oxidized to trivalent ones, and can be oxidized completely only under an atmosphere of pure oxygen. In addition, since the NCA is strongly hygroscopic, the reactions shown in the following equations occur, and normally the battery can be produced only under a humidity of less than 10%. Since NCA is prone to releasing O2, CO2, etc., the battery is easy to become swollen, and it is best manufactured as a 18650 type cylindrical battery.LiNi1-x-yCoxAlyO2+H2O→Ni1-x-yCoxAlyO+LiOH+O2 LiOH+CO2→Li2CO3 Li2CO3+HF→LiF+CO2 
Due to the structural characteristics of the material per se, the conditions for preparing NCA materials and nickel-cobalt-aluminum-lithium batteries with stable structures are very harsh. Currently, nickel-cobalt-aluminum-lithium cathode materials produced domestically still have the defects including fast capacity attenuation in the charge-discharge process, poor rate performance, and very poor storage performance. Therefore, in order to meet the requirements for mild production process, and to prepare nickel-cobalt-aluminum materials with excellent performances, it is necessary to develop a novel nickel-cobalt-aluminum precursor.
In the process for preparing a lithium nickel cobalt aluminate cathode material, as limited by calcining regime, Al3+ ions are very difficult to form a solid solution with Ni—Co at around 750° C. Accordingly, the method of solid-state mixing and sintering of separate nickel, cobalt and aluminum raw materials is seldom used. At present, Ni1-x-yCoxAly(OH)2 is generally considered as the best precursor for preparing high-performance nickel-cobalt-aluminum. Co-precipitation method is a simple and practical method for preparation and surface modification of LiNiCoAlO2. The key to co-precipitation of Ni, Co and Al is to overcome the problem that Al3+ is prone to hydrolysis to precipitate separately and therefore is difficult to form a precursor with a homogeneous structure with nickel and cobalt elements, and cannot form a high-density spherical nickel-cobalt-aluminum material. To address the problem that Al3+ is prone to hydrolysis, patent documents CN103094546A and CN103553152A proposed a method in which a complexing solution of aluminum is prepared separately as a source of aluminum, which is fed as a concurrent flow with a solution of nickel and cobalt salts, a solution of sodium hydroxide, and a solution of ammonia to prepared spherical nickel-cobalt-aluminum by controlling the crystallization. However, said method has the following problems: 1. the nickel-cobalt-aluminum precursor thus prepared usually comprises a great amount of residue sulfate ions, which are difficult to be removed by washing; 2. the aging treatment with 5 wt %-15 wt % solution of sodium hydroxide as disclosed in CN103553152A is beneficial for sulfur removal, but the washing process often leads to loss of surface aluminum element, i.e. aluminum deficiency on the surface of the prepared material, which is disadvantageous for the storage performance, the processability and the electrochemical cycling stability of the cathode material. The cycling stability, the safety performance, the processability and the storage performance of the material may be improved by increasing the doping amount of aluminum. However, the introduction of a great amount of the light metal, aluminum, would lead to reduction in the true density of the material per se and reduction in the bulk energy density of the material. In addition, the introduction of Al element, which does not have an electrical activity, would definitely lead to reduction in the energy density of the material per se. Therefore, it has become a hot spot in research to prepare a material with high energy density, high stability, excellent storage performance and processability under a relatively low doping amount of Al.
Yang-Kook Sun of Hanyang University, South Korea, developed a novel gradient material for lithium ion batteries as early as in 2008. The core of the material is a nickel-cobalt-manganese ternary material with a relatively high nickel content, and the outer coating layer is a nickel-cobalt-manganese material in which the content of nickel gradually decreases while the contents of manganese and cobalt gradually increase. Such a special cathode material shows high energy density, long service life and good safety performance. Many literatures and patent documents in China also reported methods for the preparation of cathode materials for lithium ion batteries with ingredient change of doping elements (Ni, Co, Mn, Mg, Al, Ti, Zr, etc.). Among these, the patent applications for invention CN102214819A, CN103078109A and CN103715424A all relate to co-precipitation methods for the preparation of hydroxide precursors with gradient distributions of Al element. However, in all these methods, hydroxide precursors with gradient change of aluminum element are prepared by gradually adding a solution of aluminum salt into a mixed solution of nickel and cobalt salts to control the gradual change of the aluminum concentration in the mixed solution of nickel, cobalt and aluminum salts. In the ammonia complexing system used in the methods, Al3+ hardly complexes with ammonia. Al3+ is prone to hydrolysis to form a colloid separately, so that gradient doping of Al3+ element in nickel-cobalt-manganese hydroxide cannot be achieved, which is disadvantageous for the preparation of high-density spherical precursor in which aluminum is gradiently doped.